As a class of chemical compounds, amides have demonstrated wide utility both as synthetic intermediates and end products in a variety of industrial and commercial applications. Current use includes amides as laundry anti-static agents, detergents, lubricants, foamers, and additives in products as diverse as shampoos and asphalt pavements. Sterically-hindered amides are of particular importance for use as industrial cleaners and insecticides (as described in U.S. Pat. Nos. 4,682,982 and 4,804,683, respectively).
Commercially and on industrial scales, amides are prepared via the condensation reactions of amines with acid chlorides. The later are highly-reactive acylating agents, necessitating stringent reaction control. As with many chlorinated organics, corrosion concerns warrant use of expensive glass-lined reaction vessels. An additional consideration is that the acid chloride is typically prepared by the reaction of either thionyl chloride or phosgene with the corresponding acid. Both reagents present significant toxicity and reactivity concerns which detract from the acid chloride as a useful synthetic intermediate. Moreover, under common preparatory conditions, the amide product is subject to side reactions which tend to decrease yield and introduce unwanted impurities.
An efficient, economical means for large scale amide preparation has been an on-going concern in the art. One approach which has met with some success is homogeneous catalysis, such as that described in U.S. Pat. Nos. 3,816,483 and 3,951,996. Use of a reaction-soluble Group IVb or Vb metal catalyst has alleviated many of the problems which plague more traditional synthetic routes. Generally, good yields in commercial quantities are available directly from the acid without excessive reaction times and/or unfavorable reaction conditions.
However, the prior art has associated with it a number of significant problems and deficiencies. Most are related to undesirable reaction conditions, inadequate yields, and impurities, and result from the catalytic systems currently used.
One major problem of the prior art is that amidation under homogeneous catalytic conditions is rather limited to use of ammonia and then only with a large molar excess relative to the carboxylic reactant. Generally, unacceptable results are obtained when either the acid or amine reactant is mono- or di-substituted at the .alpha.-position.
Another significant and well-documented deficiency is that Group IVb and Vb metal compounds tend to hydrolyze with loss of catalytic activity. Inasmuch as water is a by-product of acid amidation, an excessive amount of catalyst may be necessary in order to obtain acceptable yields.
A related problem associated with homogeneous catalysis is residual catalyst, the presence of which tends to haze or discolor the amide product and further contribute to the oxidative, thermal, and/or hydrolytic instability of the amide or any subsequent formulation product.
In summary, a considerable number of drawbacks and problems exist in the art relating to the preparation of amides from carboxylic acids. There is a need for an improved amidation method such that synthetically and commercially useful amides may be prepared efficiently and economically.